Retinoid carboxylic acids, the natural derivatives and synthetic analogs of vitamin A, are fundamental mediators of many biological processes, including differentiation, proliferation, and morphogenesis. The observations that retinoids have striking activity in the treatment of acute promyelocytic leukemia (APL), advanced skin and cervical cancers and head and neck cancer have generated tremendous enthusiasm for the development of retinoids for cancer treatment. However, retinoid-retinoid-resistant cancer cells and severe side effects have restricted further applications. Understanding of the molecular mechanism of retinoid action against breast cancer cell growth will enable us to enhance retinoid sensitivity an design retinoids with greater therapeutic efficacy. The effects of retinoids are mainly mediated by two classes of nuclear receptors, the retinoic acid receptors (RARs) and retinoid X receptors (RXRs). The retinoid receptors and a number of orphan receptors belong to the steroid/thyroid hormone receptor family that regulates gene expression by binding to specific DNA sequences in target genes. Research Project Molecular Mechanism of Retinoid Action focuses on mechanistic studies of the retinoid response in breast cancer cells and identification of the most effective synthetic retinoids for treatment of breast cancer. Recently, the Zhang laboratory demonstrated that RARbeta mediates the growth inhibitory effect of trans-retinoic acid (trans-RA) in breast cancer cells and subsequently, that trans-RA sensitivity I lung cancer cells is established by the dynamic equilibrium and heterodimerization of the orphan nuclear receptors COUP-TF and nur77, which regulates RARbeta expression. A pEA3-binding site in the RARbeta promoter was also identified. The proposed mechanistic studies are based on these exciting new results. We will first determine the requirement of estrogen receptor and RARalpha on RARbeta gene expression in breast cancer cells, then investigate the role of COUP-TF and nur77 in regulating trans-RA sensitivity in breast cancer cells and the mechanism leading to lack of COUP-TF expression in trans-RA-resistant breast cancer cells. The collaboration with the Leid laboratory will determine how ligand activation of nur77/RXR heterodimers regulates RARbeta promoter activity and the requirement of pEA3 in RARbeta promoter activity. The evaluation of retinoids from the Dawson laboratory will involve identification of potent agonists and antagonists for specific nuclear receptors, characterization of receptor-selective anti-AP-1 activity, and induction of apoptosis and analysis of their growth inhibitory effects against breast cancer cells. The results from these studies will enhance our understanding of the biological functions of retinoid and orphan receptors and the mechanism of retinoid resistance in breast cancer cells, and will provide a rational basis for identifying more effective selective retinoids with reduced side effects.